1. Field of the Invention
The present invention relates generally to a boom apparatus for use in lifting and transporting a payload and, more particularly, to a boom overload warning and control system for use with such a boom apparatus.
2. Discussion of the Prior Art
It is known to provide a boom apparatus comprising a plurality of relatively extensible boom sections mounted for unitary pivotal movement relative to a turret or support structure of the apparatus, and to provide appropriate means for carrying out extension of the sections and pivoting of the boom. For example, a conventional boom apparatus may include three relatively movable boom sections, a system for moving the sections axially between a retracted position and an extended position, and a hydraulic cylinder and piston extending between the boom and a support turret of the apparatus for hoisting the boom about a horizontal pivot axis between a lowered position and a raised position relative to a horizontal reference axis.
Further, such systems may employ a warning and control system in which a warning signal is generated in response to a sensed predetermined pressure level within the hydraulic cylinder which is proportional to and representative of the total moment force experienced by the boom due to the mass of the boom and any payload supported thereby. Once the predetermined pressure level is reached, the known system generates a warning and control signal which is used to indicate an overload condition of the boom apparatus and to control certain functions of the apparatus.
For example, where a hydraulic system is used to carry out the axial movement of the boom sections, the warning and control signal may be used to prevent the hydraulic system from being operable to increase the total length of the boom by only permitting the system to move the boom sections closer together. Also, the control signal may also be used to prevent the hydraulic cylinder and piston from moving to a retracted position in which the boom is lowered about its pivot axis. Both of these operations are inhibited in response to the control signal due to the potential risk of carrying out such operations when the total moment force experienced by the boom is in excess of a predetermined safe value.
In addition, if a winch is provided on the boom apparatus for lifting the payload relative to the boom, the control signal may also be used to prohibit further lifting of the payload by the winch when the pressure in the cylinder is at a level sufficient to suggest an overload condition exists. In such a condition, the winch may only be operated to lower the payload.
In this type of conventional boom overload warning and control system, the predetermined pressure level used in generating the warning and control signal represents a single moment force experienced by the boom. Thus, although the boom is capable of supporting various maximum loads depending upon the total length of the boom and the angle of the boom relative to horizontal, the warning and control system is limited to just one value. Because of this limitation in the known systems, the pressure level at which the warning and control signal is generated represents the lowest maximum moment load under which the boom apparatus is operable in order to insure that the boom apparatus never exceeds that particular overload condition.
Since this lowest maximum pressure or moment load corresponds to pressures at longer boom lengths, the utility of the boom is reduced at shorter boom lengths since the actual maximum moment load that is capable of being supported by the boom at the shorter boom lengths is never permitted to be handled by the apparatus due to actuation of the warning and control system at the relatively low pressure level.